Xylylenediamine (bis(aminomethyl)benzene) is a useful starting material, for example for the synthesis of polyamides, epoxy hardeners or as an intermediate for preparing isocyanates.
The term “xylylenediamine” (XDA) comprises the three isomers ortho-xylylenediamine, meta-xylylenediamine (MXDA) and para-xylylenediamine.
The term “phthalonitrile” (PN) comprises the three isomers 1,2-dicyanobenzene=o-phthalonitrile, 1,3-dicyanobenzene=isophthalonitrile=IPN and 1,4-dicyanobenzene=terephthalonitrile.
The phthalonitriles are solids (for example isophthalonitrile (IPN) melts at 161° C.) and have relatively poor solubilities in organic solvents.
The two-stage synthesis of xylylenediamine by ammoxidation of xylene and subsequent hydrogenation of the resulting phthalonitrile is known.
Unconverted dinitriles can be removed by distillation from the XDA only with great difficulty.
U.S. Pat. No. 4,482,741 (UOP Inc.) describes the hydrogenation of PN in the presence of ammonia, a specific catalyst and XDA as a solvent.
In MXDA, the solubility of IPN at 70° C. is approx. 20% by weight.
EP-A2-1 193 247 and EP-A1-1 279 661 (both Mitsubishi Gas Chem. Comp.) relate to a process for purifying isophthalonitrile (IPN) and to a process for preparing pure XDA.
EP-A2-1 193 244 (Mitsubishi Gas Chem. Comp.) describes a process for preparing XDA by hydrogenating phthalonitrile which is synthesized in a preceding stage by ammoxidation of xylene, the vaporous product of the ammoxidation stage being contacted directly with a liquid organic solvent (quench) and the resulting quench solution or suspension being sent to the hydrogenation.
Preferred organic solvents are C6-C12 aromatic hydrocarbons such as xylene and pseudocumene (column 6, paragraph [0027] and [0028]).
U.S. Pat. No. 3,069,469 (California Research Corp.) teaches, as a solvent for the hydrogenation of aromatic nitriles such as PN, aromatic hydrocarbons, xylene, dioxane and aliphatic alcohols.
DE-A-21 64 169 (Mitsubishi Gas Chemical Co., Inc.) describes, at page 6, last paragraph, the hydrogenation of IPN to meta-XDA in the presence of an Ni and/or Co catalyst in ammonia as a solvent.
GB-A-852,972 (equivalent: DE-A-11 19 285) (BASF AG) discloses the use of ammonia and XDA as solvents in the hydrogenation of PN. The reactant solution is prepared starting from solid PN in an extra step in a separate vessel (cf. page 2, lines 119-120).
JP-A-2003-327563 (Mitsubishi Gas Chem. Co., Inc.) relates to a process for the fixed bed hydrogenation of aromatic dinitriles which are used in the form of 1-10% by weight solutions.
The six patent applications WO-A-05/028417, WO-A-05/026102, WO-A-05/026103, WO-A-05/026104, WO-A-05/026100 and WO-A-05/026101 (BASF AG) each relate to processes for preparing XDA.
In the different processes for preparing phthalonitrile, it is obtained as a solid or dissolved in a solvent, for example pseudocumene, or as a melt. The handling of solvents is typically difficult and laborious. Owing to the low solubility of phthalonitrile in solvents such as o-xylene, m-xylene, p-xylene, pseudocumene, mesitylene, ethylbenzene or methylpyridine, the further processing in a solvent entails very large amounts of solvent which generally have to be removed by distillation after the hydrogenation, which, in accordance with the large streams, entails large apparatus and high energy demands.
Alternatively, an extraction of the PN with water with subsequent distillation is possible. Here too, the energy demands are high, since the water has to be distilled off and the solvent regenerated, at least in a substream.
WO-A-05/026098 (BASF AG) relates to a process for preparing XDA by continuously hydrogenating phthalonitrile over a heterogeneous catalyst in the presence of liquid ammonia in a reactor, in which a portion of the reactor effluent is recycled continuously to the reactor inlet as a liquid circulation stream (circulation mode), in which, by means of a mixing unit, a stream of a phthalonitrile melt is conducted in liquid form into the circulation stream around the hydrogenation reactor, the phthalonitrile conversion in the reactor in single pass being greater than 99% and the circulation stream consisting to an extent of greater than 93% by weight of liquid ammonia and xylylenediamine and not comprising any further solvent for phthalonitrile.
WO-A-05/026099 (BASF AG) relates to a process for preparing XDA by continuously hydrogenating PN over a heterogeneous catalyst in the presence of liquid ammonia in a reactor, a stream of a phthalonitrile melt being mixed in liquid form with a stream of liquid ammonia by means of a mixing unit and the liquid mixture being conducted into the hydrogenation reactor.
Owing to the high melting points, phthalonitriles can be handled in molten form only with difficulty and with a considerable level of complexity for protective heating. For conveying against high pressure, as required in the procedure according to the abovementioned WO-A-05/026098, special pumps are used, for example heatable high-pressure membrane pumps which are firstly very prone to faults and secondly very expensive. However, the process according to WO-A-05/026098 has the advantage that only a comparatively small amount of liquid ammonia has to be worked up, specifically the amount which is discharged with the xylylenediamine reaction product from the reaction stage for workup. The amount of ammonia in the reactor can be selected by means of appropriate selection of the size of circulation, in spite of a small fresh ammonia stream, such that the reaction can proceed under optimal conditions (for example amount of NH3 in mol per mole of phthalonitrile used).
In contrast, in a procedure analogous to the abovementioned WO-A-05/026099, the conveying of the phthalonitrile melt necessitates merely a heatable pump which has to convey the melt into the mixing vessel which is operated at distinctly lower pressure, for example from 20 to 40 bar. To this end, it is possible, for example, to use a less expensive and less fault-prone multistage centrifugal pump. To convey the solution at reactor pressure (e.g. 200 bar) an additional pump is required, but the handling of the solution of phthalonitrile is distinctly simpler and the requirements on the protective heating are considerably smaller. Accordingly, a less expensive pump can be used which is less fault-prone at the lower operating temperature. Especially in the case of shutdowns or operational faults, for example in other parts of the plant too, it is possible to handle the solution better than the melt. However, there is the disadvantage that the solubility of phthalonitrile in ammonia is restricted and temperature-dependent. At lower temperature, the achievable phthalonitrile concentration in ammonia is small but the dissolution vessel can be operated at low pressure, which is also associated with correspondingly low demands on the melt pump. However, this also makes it necessary for a large amount of ammonia, after the hydrogenation of the phthalonitrile, to be worked up again and recovered by pressure distillation. When the temperature in the dissolution vessel is increased, it is possible to set a larger phthalonitrile concentration and less ammonia accordingly has to be worked up. On the other hand, this raises the pressure in the dissolution vessel and the demands for the melt pump and the protective heating rise. The apparatus, machines and equipment become costlier and more fault prone.